B-tagging

Importance

• The physics of bottom quarks is quite interesting; in particular, it sheds light on CP violation.
• Some important high-mass particles decay into bottom quarks. Top quarks very nearly always do so, and the Higgs boson is expected to decay into bottom quarks more than any other particle given its mass has been observed to be about 125 GeV. Identifying bottom quarks helps to identify the decays of these particles.

  Methods

• Hadrons containing bottom quarks have sufficient lifetime that they travel some distance before decaying. On the other hand, their lifetimes are not so high as those of light quark hadrons, so they decay inside the detector rather than escape. The advent of precision silicon detectors within particle detectors has made it possible to identify particles that originate from a place different to where the bottom quark was formed, and thus indicating the likely presence of a b-jet.
• The bottom quark is much more massive than anything it decays into. Thus its decay products tend to have higher transverse momentum. This causes b-jets to be wider, have higher multiplicities and invariant masses, and also to contain low-energy leptons with momentum perpendicular to the jet. These two features can be measured, and jets that have them are more likely to be b-jets.
• Opposite-side algorithms have been used at the LHCb to tag the flavor in pairs of b quarks using the decay products of B-hadrons to infer the flavor of B-mesons.